1. Field of the Invention
The invention relates to a system and method for controlling ratio changes for a multiple-ratio transmission in a vehicle driveline having an engine with an electronic throttle control.
2. Background Art
Automatic transmissions used in contemporary vehicle powertrains include multiple-ratio gearing wherein the torque flow paths through the gearing elements and the relative speeds of the gearing elements are controlled by fluid pressure-operated friction elements; i.e., friction clutches and brakes. The management of the torque transfer through the gearing from the throttle-controlled engine to the traction wheels is achieved by a control system having an electronic controller that responds to powertrain variables, including engine variables and driver commands.
The control system develops clutch and brake control pressure, which is determined by solenoid valves under the control of the controller. Typically, a fluid pressure circuit would include a solenoid valve dedicated to circuit pressure control, and a separate solenoid valve circuit to effect control of the pressure-operated clutches and brakes during upshifts and downshifts.
In the transmission disclosed in application Ser. No. 09/636,729, filed Aug. 10, 2000, now U.S. Pat. No. 6,385,520, a separate sulenoid valve is provided for controlling each ratio shift, the offgoing friction element being activated independently of the ongoing friction element during a ratio change.
Copending patent application Ser. No. 09/366,416, filed Aug. 4, 1999, now U.S. Pat. No. 6,253,140, describes engagement control logic wherein the ratio changes occur with an adaptive engagement feel. The controller for this transmission control provides consistent, smooth engagement of a friction element during vehicle startup. It includes a controller with an adaptive strategy that maintains consistent shift quality and takes into account system variations, such as changes in coefficient of friction. friction element wear, etc. Actual slip rate during engagement of die friction element is maintained at a desired value.
Each of these copending patent applications is assigned to the assignee of the present invention.
In prior art systems corresponding to those described in the copending patent applications, the control strategy for the friction elements involves a direct relationship between the transmission output torque control and the ratio change time control. This may require a compromise in shift quality because optimum transmission output torque control is not necessarily consistent with optimum ratio shift timing control. The control strategy of known transmission systems for controlling friction element capacity must be changed rapidly in a controlled fashion to match varying engine torque. The friction elements may not have a response to control commands that is fast enough to accomplish a smooth ratio change, which results in lower shift quality. Furthermore, variations in shift quality on a shift-to-shift basis may occur.
Another related copending application, which also is assigned to the assignee of the present invention, is Ser. No. 09/665,353, filed Sep. 18, 2000, now U.S. Pat. No. 6,278,926. It discloses a multiple-ratio gear system in which independent pressure control of separate friction elements is achieved using separate variable force solenoids. A microprocessor receives input continuously from driveline sensors and stores and executes a control logic for controlling the oncoming friction element during a ratio change. Information that is learned from the input signals is executed on a real-time basis to calculate clutch pressures to achieve optimum shift quality.
The disclosures of the copending patent applications are incorporated in the present disclosure by reference to supplement the present disclosure.
In each of the systems disclosed in the copending patent applications, the slip time during a ratio change is controlled by controlling friction element pressure. That is, the controller controls the solenoid valves that determine friction clutch pressure.
The control system and method of the invention uses a closed-loop controller to achieve the desired ratio rate during a shift from one gear ratio to the next. The ratio rate is a function of accelerator pedal position. Concurrently, the transmission output torque is controlled using a friction element capacity control strategy. The friction element capacity control involves a determination of the desired wheel torque using a torque feed-forward term. This provides a quick response to changes in transmission input torque.
The accelerator pedal in the system of the invention is decoupled from the engine throttle. This is achieved by using an electronic throttle control, under the control of the vehicle operator, which is not mechanically connected directly to the engine throttle. Control logic is used to interpret a driver demand for torque by sensing the movement of the accelerator pedal. Controlling the input torque to a manageable level results in a smooth torque transition during a ratio change and reduces variation in shift quality from one shift to the next.
Engine torque is controlled using the system and method of the invention by using a closed-loop controller to achieve a desired ratio change rate, as mentioned above. The controller establishes a commanded ratio rate during a ratio change interval and compares it to the actual ratio rate to detect an error. A commanded engine torque then is established, which reduces the error.
Torque capacity control of the friction elements, unlike the closed-loop control of the engine torque, is an open-loop control in which the desired wheel torque is determined by the controller as speed inputs are received by the controller from an engine speed sensor, a turbine speed sensor and an output shaft speed sensor. The controller establishes a desired vehicle wheel torque as a function of accelerator pedal position. It calculates friction element pressure to achieve optimum friction element capacity during a ratio change interval.
A transmission that would embody the control logic of the invention may include a hydrokinetic torque converter in which the impeller is driven by the engine. The converter turbine would be coupled directly to the output shaft through multiple-ratio gearing.
The system of the invention includes sensors for measuring engine speed, turbine speed, torque output shaft speed and accelerator pedal position. An electronic controller in the system has a first portion in communication with the sensors. A processor unit calculates a commanded ratio rate and an actual ratio rate during a ratio change interval and determines an error between the commanded ratio rate and the actual ratio rate. A second portion of the electronic controller has a processor for calculating engine torque. as a function of the ratio rate error. A third portion of the processor calculates engine control parameters in a closed-loop fashion as a function of the commanded engine torque.
Another aspect of the invention is a ratio change control method wherein engine speed, turbine speed and torque output shaft speed are measured as other steps are carried out including calculating driver demand for engine torque, commanded and actual ratio rates during a ratio change interval, desired traction wheel torque as a function of the driver demand for engine torque, friction element pressure in an open-loop fashion based on the desired traction wheel torque, determining an error between the commanded ratio rate and the actual ratio rate, calculating a commanded engine torque as a function of ratio rate error, and calculating engine control parameters in a closed-loop fashion as a function of the driver demand for engine torque.